The coating of electrically conductive substrates by electrodeposition is a well known and important industrial process. (For instance, electrodeposition is widely used in the automotive industry to apply primers to automotive substrates). In this process, a conductive article is immersed as one electrode in a bath of a coating composition of an aqueous emulsion of film-forming polymer. An electric current is passed between the article and a counter-electrode in electrical contact with the aqueous emulsion, until a desired coating is produced on the article. In a cathodic electrocoating process the article to be coated is the cathode and the counter-electrode is the anode.
Resin compositions used in the bath of cathodic electrodeposition process are also well known in the art. These resins are typically made from polyepoxide resins which have been chain extended and then an adduct is formed to include a nitrogen in the resin. Nitrogen is typically introduced through reaction with an amine compound. Typically these resins are blended with a crosslinking agent and then neutralized with an acid to form a water emulsion which is usually referred to as a principal emulsion.
The principal emulsion is combined with a pigment paste, coalescent solvents, water, and other additives (usually at the coating site) to form the electrodeposition bath. The electrodeposition bath is placed in an insulated tank containing the anode. The article to be coated is made the cathode and is passed through the tank containing the electrodeposition bath. The thickness of the coating is a function of the bath characteristics, the electrical operating characteristics, the immersion time, and the like.
The coated object is removed from the bath after a given amount of time. The object is rinsed with deionized water and the coating is cured typically in an oven at sufficient temperature to produce crosslinked coating.
The prior art of cathodic electrodepositable resin compositions, coating baths, and cathodic electrodeposition processes are disclosed in U.S. Pat. Nos. 3,922,253; 4,419,467; 4,137,140; and 4,468,307 which are incorporated herein by reference.
The pigment dispersant is an important part of an electrocoat primer composition. The dispersion process involves the separation of the primary pigment particles from their agglomerates or aggregates, the displacement of occluded air and absorbed water, and the wetting and coating of the pigment surfaces with the dispersion resin. Ideally, each primary particle, having been mechanically separated during dispersion, is also stabilized against flocculation. If the pigment particles are not properly dispersed and stabilized in the paint, the advantages built into the pigment by the manufacturer may be lost. For instance, the pigment may settle in the electrodeposition bath which can result in loss of corrosion protection of the substrate In addition, surface appearance, operating characteristics and the like may be adversely impacted by inadequate pigment dispersion.
All previous pigment pastes for cathodic electrocoat processes contain water to reduce the viscosity and to make the paste easier to grind. (U.S. Pat. No. 4,110,292 discloses using a solvent based pigment paste which is incorporated into a non-electrocoat latex paint). However, pigment pastes containing water can cause foaming in the pigment paste and/or flocculation of the pigment under certain circumstances. Foaming can be a serious problem for the following reasons: (1) foaming makes it more difficult to grind pigment; (2) foaming makes it difficult to check the pigment particle size; (3) foaming makes it more difficult to unload the pigment paste from the grinding mill; and (4) foaming can introduce surface defects on the film or coating deposited by the electrocoating process.
An additive can be used in an attempt to solve a foaming problem. However, there are no satisfactory defoamers for use in making water based pigment pastes. Furthermore, the defoamers that are available can cause corrosion problems and adhesion loss of the topcoat to the electrocoated film.
What is needed is to eliminate or substantially reduce foaming during the process for making pigment pastes which are used in a cathodic electrocoating bath.